Although local oxidation of silicon (LOCOS) has been widely used as an element isolation technology in an LSI production process, the introduction of a new element isolation technology is being developed to facilitate the downsizing of a semiconductor element.
Shallow groove isolation (SGI) which is effected by burying an insulating film, such as a silicon oxide film, in a trench formed in a silicon substrate can (a) reduce the interval between, (b) easily control the thickness of an element isolating film and set a field reverse voltage, and (c) separate an anti-reflection layer from a diffusion layer and a channel region by striking different impurities into the side wall and the bottom of the trench. Therefore, it is more advantageous in securing sub-threshold characteristics and reducing a bonding leak and back-gate effect than the local oxidation of silicon.
A general method of forming an element isolation trench is as follows. A silicon substrate is first thermally oxidized to form a thin silicon oxide film on the surface, a silicon nitride film is formed on the silicon oxide film by chemical vapor deposition (CVD), and the silicon nitride film of an element isolation region is removed by dry etching using a photoresist film as a mask. Thereafter, the photoresist film is removed, a trench as deep as 350 to 400 nm is formed in the substrate by dry etching using the silicon nitride film as a mask, and the substrate is thermally oxidized to form a thin silicon oxide film on the inner wall of the trench. This silicon oxide film is formed to eliminate etching damage which occurs on the inner wall of the trench and to alleviate the stress of the silicon oxide film buried in the inside of the trench in a later step.
After a thick silicon oxide film is formed on the substrate containing the inside of the trench by CVD, the substrate is heated to finely densify the silicon oxide film buried in the inside of the trench. Thereafter, the silicon oxide film formed on the silicon nitride film is removed by chemical mechanical polishing (CMP) so that the silicon oxide film remains only in the inside of the trench, and the unnecessary silicon nitride film is removed by etching to complete an element isolation trench.
It is known that, in the above element isolation structure, a gate oxide film formed on the surface of the substrate of an active region is locally thin at the end portion (shoulder portion) of the active region and the field of gate voltage is concentrated upon this shoulder portion with the result of the occurrence of a phenomenon in which a drain current flows with a low gate voltage (may be called “kink characteristics” or “hump characteristics”). As a solution to this, a technology for rounding the shoulder portion of the active region is proposed.
For example, Japanese Patent Laid-open No. Sho 63-2371 indicates such a problem that, when a fine MISFET having a channel width of 1 μm or less is formed in the active region of the substrate surrounded by the above element isolation trench, it cannot be used as a device due to a reduction in threshold voltage (Vth), so-called “narrow channel effect”. This is because the shoulder portion of the active region has an angular cross section close to a right angle in the element isolation structure where an insulating film is buried in the inside of a trench formed in the substrate, whereby the field of the gate voltage is concentrated upon this region and a channel is formed with a low gate voltage.
The above publication discloses a technology for preventing a reduction in threshold voltage by forming a trench in the substrate, rounding the shoulder portion of the active region through wet oxidization at 950° C. and thickening the gate oxide film of the shoulder portion of the active region in order to suppress the above narrow channel effect.
Japanese Patent Laid-open No. Hei 2-260660 also discloses a technology for suppressing the concentration of the field of the gate voltage upon the shoulder portion of the active region by rounding the shoulder portion to prevent the occurrence of the above kink (hump) characteristics. In this publication, the shoulder portion of the active region is substantially rounded by the following method.
The element formation region of a semiconductor substrate is covered with a mask made of a laminate film consisting of an oxide film and an oxidation resistant film, and the substrate is thermally oxidized in this state to form an oxide film on the substrate of an element isolation region such that one end of the oxide film encroaches on the element formation region. Thereafter, the oxide film of the element isolation region is removed by wet etching using the above oxidation resistant film as a mask, a trench is formed in the substrate of the element isolation region by reactive ion etching using the above oxidation resistant film as a mask, the substrate is thermally oxidized to form a thermal oxide film on the inner wall of the trench, and the shoulder portion of the trench is rounded.